1. Field of the Invention
The present invention relates to a digital radio system which forms a high speed digital radio transmission line through a plurality of repeater offices positioned within line-of-sight distances using a microwave band radio frequency, more particularly relates to repeater equipment and end equipment respectively constituting a repeater office and end office in that system and to a receiver mounted in that equipment.
2. Description of the Related Art
The striking advances being made in digital radio transmission technology have led to the use of digital radio systems using microwave band radio frequencies for tandem regenerative repeat operations in both communication lines for domestic trunks and communication lines used for international communication.
In such a digital radio system, as will be explained in detail later with reference to the drawings, there have been the following problems:
(i) When the receiving end recognizes that the bit error rate (BER) is above a certain threshold, a trial of parallel connecting transmission is started, and therefore if a notch occurs successively between adjoining radio transmission lines or the notch causes significant fluctuations in the received field strength, it becomes impossible to secure a sufficient transmission quality for communication services. A situation then arises where the lines are blocked or calls have to be disconnected under predetermined communication control protocols and as a result the quality of the service deteriorates. PA1 (ii) Accordingly attempts at parallel connecting transmission are started after the elapse of a long period of a few tens of milliseconds from the time of occurrence of a notch, so in many cases a large amount of transmission information is sent over the working radio transmission line with a deteriorated transmission quality before the parallel connecting transmission begins and the quality of the transmission temporarily becomes extremely poor. PA1 (iii) When a similar notch occurs at another working radio transmission line after the bit error rate (BER) once exceeds the above threshold, since the standby radio transmission line is not released until the above bit error rate returns to below the threshold, the attempt at parallel connecting transmission will be suspended and the quality of transmission will remarkably fall in many cases. PA1 (iv) When a notch grows at a remarkably high speed in the transmission band of the working radio transmission line, the signature changes rapidly along with the notch growth. Therefore, in the related art where the fact of the bit error rate exceeding the threshold was detected based on the result of a parity check, it was not possible to change over from the working radio transmission line to the standby radio transmission line uninterruptedly.